Acrylated polyurethanes are well known for use in the buffer coating of optical glass fiber. These polyurethanes are typically formed by reacting polyoxyalkylene ether glycols with organic diisocyanates to form isocyanate-terminated polyurethanes which are capped with a monohydric acrylate, such as 2-hydroxyethyl acrylate. Various polyoxyalkylene ether glycols can be used, and it is preferred to employ those in which the repeating unit is a tetramethylene ether group (derived by reacting a polyoxytetramethylene glycol with the diisocyanate) because the tetramethylene ether group is more resistant to water than the corresponding ethylene ether or propylene ether group.
Unfortunately, the use of polyoxytetramethylene glycol introduces undesired crystallinity which causes the coatings to become harder and less flexible when low service temperatures are encountered. This crystallinity need only pervade portions of the coating in order to cause difficulty. This leads to microbending difficulties which are unacceptable. Thus, it is desired to take advantage of the superior water resistance conferred through the use of polyoxytetramethylene glycol, but this opportunity is limited by the undesired crystallinity it introduces.